Rikiune said: I remember that fight. Stuff was tough.
There's nothing difficult in this fight except attempts to draw Demi at low levels - it fails so often with low Mag stat. You should Blind him and then stock some Demi, because if you use it from your own stock and not with command Draw - Cast, Diablos will counterattack with Curaga, which heals about 3000 hp. Just hit him with Demi and use someone's limit when GF's hp will be low enough.